docs: add comprehensive API documentation for programmatic usage

[MichaelKreil]

Summary

• Added comprehensive API documentation in API.md for developers who want to use xst programmatically in their Node.js applications
• Updated README.md to include a "Programmatic Usage" section linking to the new API documentation
• Provides complete examples for all commands (upload, get, list, exec, rm, edit, info, package management)
• Includes utility function documentation, TypeScript support guide, and best practices

What's Included

New File: API.md

A comprehensive 800+ line API documentation covering:

• Quick Start - Get up and running quickly
• Connection Configuration - 4 different ways to configure database connections (programmatic, environment variables, config files, utilities)
• Command API Reference - Full documentation for all 8 commands with parameters and examples
• Utility Functions - Documentation for connection, error handling, messaging, sizing, versioning, and glob utilities
• TypeScript Support - How to use JSDoc types with TypeScript
• Examples - 5 complete real-world examples:
  • Complete upload workflow with admin verification
  • Backup and restore operations
  • Bulk operations (cleanup old files)
  • Package management automation
  • Best practices
• Best Practices - Production-ready guidelines

Updated: README.md

• Added "Programmatic Usage" section between the command table and CLI examples
• Includes a quick example showing how easy it is to use xst as a library
• Links to the comprehensive API documentation

Motivation

This addresses issue #5 from the improvement recommendations - adding API documentation for programmatic usage. While xst works great as a CLI tool, it can also be used as a library for automation, CI/CD pipelines, backup scripts, and other Node.js applications. This documentation makes that use case accessible to developers.

Test Plan

• Verify API.md renders correctly on GitHub
• Check that all code examples use correct syntax
• Confirm README.md link to API.md works
• Review that examples align with actual command signatures
• Validate TypeScript type references are accurate

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